Thermionic power conversion triode



May 31, 1966 M. GOTTLIEB ETAL THERMIONIC POWER CONVERSION TRIODE wnwessss: fimmg R4;

Filed June 27, 1961 HEAT SOURCE Fig.|.

INVENTORS Milton Go'nlieb and Robert J. ZoHweg.

ATTORNEY United States Patent THERMIONIC POWER CONVERSION TRIODE Milton Gottlieb, Forest Hills, and Robert J. Zollweg,

Monroeville, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed June 27, 1961, Ser. No. 120,073 2 Claims. (Cl. 310-4) This invention relates to thermionic power conversion devices and, more particularly, to thermionic power conversion devices employing a plasma for space charge neutralization.

Thermionic power conversion devices directly convert thermal energy into electrical energy by use of thermionic electron emission. In the operation of thermionic conversion devices, one of the principal problems is to neutralize the space charge. Several known schemes include provision for very small cathode to anode spacing, use of crossed electric and magnetic fields, and use of an ionizable gas. Although successful devices have been described wherein a close spacing is accomplished, it is still the case that it is generally difficult to maintain extremely small spacings over a sufficiently large area at the necessary operating temperature. The use of crossed electric and magnetic fields appears to be limited at high currents because of electron-electron scattering. The most popular approach to space charge neutralization is by the use of a plasma generally using cesium gas ionized by contact ionization with a hot, high work function surface on the electron emitter.

Space charge neutralization by a plasma has been limited to the use of cesium or some other alkali metal gas whose ionization potential is low. Unfortunately such materials are often corrosive. Furthermore, a'high work function cathode must be used to ionize the gas or some other suit-able surface must be provided in the device. Relatively high pressures are required in order to provide the necessary high ion densities resulting in greater losses associated with the gas. of the cell must be maintained at a controlled high temperature to prevent condensation of the gas on the envelope wall.

It is, therefore, an object of the present invention to provide a thermionic power conversion device wherein greater variety in the selection of a gas for space charge neutralization is possible.

Another object is to provide a thermionic conversion device wherein the spacing between the anode and cathode is not critical.

Another object is to provide a thermionic power conversion device wherein a low work function cathode may be used.

Another object is to provide a power conversion device from which either A.C. or DC. power may be obtained readily.

According to the invention there is provided a power conversion device including a thermionic source of electrons, an electron collector spaced from the source and means, such as an accelerating electrode to accelerate the electrons from the source to an energy sufficient to ionize, by impact ionization, a gas disposed in the cell and thereby neutralize the space charge.

Features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention together with the abovementioned and further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which: v

FIGURE 1 is a cross-sectional view of one embodiment Also, the envelope Patented May 31, 1966 of the thermionic power conversion device in accordance with the present invention; and,

FIG. 2 is a cross-sectional view of an alternative embodiment of the present invention.

Referring now to FIG. 1 there is shown within an evacuated envelope 10 a first electrode 12 which has an electron emissive surface and which is disposed within an indentation 11 of the envelope 10 so that a heat source 14 may be readily applied thereto. A second electrode 16 is substantially parallel to the emitter 12 for collection of electrons from the emitter. A control electrode or accelerating electrode 18 is disposed around the path between the emitter 12 and the collector 16. In the external circuit, the accelerating electrode 18 ismaintained at a somewhatpositive potential relative to that of the emitter 12 by a bias potential source 20. Means, such as a load resistor 22, is provided across which the output from the emitter '12 and collector 16 can be derived. It will be obvious that the structure shown in FIG. 1 is merely schematic and exemplary of possible arrangements which may be used in accordance with this invention.

The device includes a body 24 of a suitable vaporizable material or, alternatively, merely contains a material which is a vapor even at room temperature. Means, such as a permanent magnet 26, are provided external to the .envelope 10 for formation of a magnetic field.

In operation, the electrons from the emitter 12 are accelerated toward the accelerating electrode 18. Due to the magnetic field, which is not absolutely necessary but is preferable, electrons are confined to flow in axial helices so that only a very small fraction of the electrons are collected by the accelerating electrode 18. Also, the accelerating electrode 18 may have a secondary emission ratio of about unity at the accelerating potential used so that the net loss of electrons is very small. Electrons,

after passing through the accelerating electrode 18, are

decelerated and only those having sufiiciently high energy will reach the collector 16. Those which pass through the accelerating electrode 18 but which do not reach the collector 16 are accelerated back toward the emitter 12. If a gas is present in the cell, it will be highly ionized by the energetic electrons, especially in the region of the accelerating electrode 18, hence resulting in neutralization of the space charge which builds up between the electrodes especially near the collector 16.

The important advantages of the present invention stem from the fact that with a relatively low potential of about 20 volts on the accelerating electrode, the velocity of the accelerated electrons is over a factor of ten greater than the velocity of the thermal electrons so that only the ion density is required for space charge neutralization as in those instances in which contact ionization is used. Also to produce the necessary ions by impact requires lower gas pressure than for contact ionization.

Among the alternative structures Which may be employed in accordance with this invention is that shown 'in FIG. 2 in which the emitter 112 is centrally disposed with a channel 111 therein for the introduction of a heat bearing medium. The channel 111 is essentially a reentrant portion continued all the way through the envelope 110. The emitter '112 is emissive from two opposite'sur-faces and two grid-like collectors 116 and 116, and accelerating electrodes 118 and 118 are provided within the same device. It will be noted that the accelerating electrodes 118 and 118' are in a different relative position than the accelerating electrode 18 in FIG. 1. Here the accelerating electrodes 118 and 118 are remote from the emitter 112 rather than between it and the collectors 116 and 116'. This arrangement is suitable so long as the essential function is performed of accelerating electrons from the emitter 112 to sufficiently high energy to produce adequate ionization in the gas. Of course, a configuration such as that shown in FIG. 2 may be used wherein the accelerating electrodes are the grids 116 and 116 and the collectors are the electrodes indicated as 118 and 118'. Other modifications are obviously possible.

The previously described structures are also capable of power generation in a vacuum in which case the function of the accelerating electrode is simply to accelerate electrons from the cathode to the anode. However, space charge problems are encountered in such devices, and spacing between the electrodes becomes critical.

It is desirable that voltage be applied to the accelerating electrode 18, 118 or 118' without the consumption of power which is a very large fraction of the total output power of the device. For this purpose a low current is desirable in the accelerating circuit which demands that the area of the accelerating electrode be kept at a minimum. This may be achieved by the use, in a device such as that shown in FIG. 1, of a helix of very fine wire serving as the accelerating electrode 18.

Therefore,- there has been shown thermionic power conversion devices comprising three electrodes wherein one is for the purpose of accelerating electrons to produce ionization by impact with gas atoms. The use of a magnetic field helps reduce the power dissipated in the accelerating electrode circuit by preventing electrons from being collected by this electrode.

As in prior art devices, a low ionization potential gas, such as an alkali metal gas, may be used. However, operation is not limited to such gases and the choice of a gas is primarily determined by the desire for obtaining a high ionization efficiency and a low scattering cross section. As a further improvement a gas may be selected which has a high photo ionization so that the ionization produced by electrons need not be particularly great.

An example of a gas which has a high ionization efficiency is one of neon with about 1% of argon therein. This occurs due to the high probability of ionization of the neon by metastable argon atoms. This results in a gas mixture having an ionization efliciency higher than that of either of the gases separately.

Compared to devices wherein the gas is ionized by contact With a hot cathode, pressures in the devices in accordance with this invention may be of the order of of that necessary in the previous devices.

A low boiling point gas may be selected avoiding the necessity of temperature control on the envelope of the device.

- i If the voltage applied to the accelerating electrode is varied in time, a pulsating D.C. output is obtained which can be utilized as AC. power.

While the present invention has been shown and described in certain forms only, it will be obvious to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the spirit and scope thereof.

We claim as our invention:

1. A thermionic power conversion device comprising: a thermionic source of electrons, an electron collector spaced from said source, an accelerating electrode having a configuration such that it encircles the path between said source and said collector, means to maintain said accelerating electrode at a potential more positive than that of said source, an ioniza ble gas disposed between said source and said collector, means to provide a magnetic field to axially restrict the electrons from said source to reduce the number thereof collected by said accelerating electrode and means for deriving an electrical output from said collector and said emitter.

2. A thermionic power conversion device comprising: a thermionic source of electrons, an electron collector and means for neutralizing space charge between said source and said collector, said means comprising an ionizable gas disposed within said envelope, an accelerating electrode having a configuration such that it encircles the path between said source and said collector for accelerating electrons from said source to sufficient energy to produce ions in said gas by impact ionization to prevent accumulation of space charge between said source and said collector, means for providing a magnetic field to axially restrict electrons from said source to reduce the number thereof collected by said accelerating electrode.

References Cited by the Examiner UNITED STATES PATENTS 5/1958 Boutry et al. 313-63 6/1962 Peters et a1. 3104 MILTON O. HIRSHFIELD, DAVID X. SLINEY,

Examiners.

I. HINKLE, Assistant Examiner. 

1. A THERMIONIC POWER CONVERSION DEVICE COMPRISING: A THERMIONIC SOURCE OF ELECTRONS, AN ELECTRON COLLECTOR SPACED FROM SAID SOURCE, AN ACCELERATING ELECTRODE HAVING A CONFIGURATION SUCH THAT IT ENCIRCLES THE PATH BETWEEN SAID SOURCE AND SAID COLLECTOR, MEANS TO MAINTAIN SAID ACCERLATING ELECTRODE AT A POTENTIAL MORE POSITIVE THAN THAT OF SAID SOURCE, AN IONIZABLE GAS DISPOSED BETWEEN SAID SOURCE AND SAID COLLECTOR, MEANS TO PROVIDE A MAGNETIC FIELD TO AXIALLY RESTRICT THE ELECTRONS FROM SAID SOURCE TO REDUCE THE NUMBER THEREOF COLLECTED BY SAID ACCELERATING ELECTRODE AND MEANS FOR DERIVING AN ELECTRICAL OUTPUT FROM SAID COLLECTOR AND SAID EMITTER. 